Normally, the shrubs and grasses of the tundra trap the snow in the winter and prevent it from blowing into the landscape. But as temperatures rise, taller shrub species become more abundant, trapping thicker layers of snow. It might sound awesome – all that snow keeps the permafrost from warming up – but in fact, it keeps the winter cold from penetrating the ground enough to keep it frozen. And that’s a problem, because if the permafrost doesn’t cool enough to stay frozen – well, permanently – it will start releasing this trapped carbon dioxide and methane, an extremely potent greenhouse gas.
“In other cases, shrubs are darker than grasses, which changes the albedo,” says Kerby, referring to how the landscape reflects light back into space. White snow reflects light, while darker bare earth and green plants absorb it. “It’s kind of like wearing a black T-shirt on a summer day versus a white T-shirt: you’re just going to feel warmer, because black absorbs more heat,” Kerby continues. “And that will melt the snow faster, or it can thaw the permafrost faster.”
To make the arctic carbon cycle even more complicated, all this vegetation of course sequesters carbon: plants suck CO2 and cough up oxygen. “So one of the big questions is, will this greening signal, these increases in plants, compensate for the carbon losses from systems as the permafrost thaws?” says Isla Myers-Smith, an environmentalist at the University of Edinburgh, who is overseeing the research and co-authored the article.